Unconventional impurity band conduction and carrier dynamics in Ni substituted FeSi.

Analysis of the resistivity, thermoelectric power, and heat capacity of FeNiSi is presented in this report. In-spite of Ni having two extra valence electrons as compared to Fe, the physical properties are observed to be dominated by holes. In this report, we have explained this unusual hole dominant scenario by a modified two narrow-band model. According to this model, the impurity electrons which are nearer to conduction band get shifted towards lower energy level thereby leaving holes around the Fermi level, and hence a hole dominated scenario at low temperatures. Due to this hole like density of states around the Fermi level, the nickel substitution could only produce a weak ferromagnetic behavior. Such a picture may assist in understanding the thermopower of similar systems i.e. Ni substituted on Fe site, such as FeNiVAl. We have also found that the activation energy derived from resistivity and thermoelectric power decreases with increasing Ni concentration.